Defining Terms: Paraformaldehyde, Aqueous Formaldehyde, and Formaldehyde
Commercially, formaldehyde is available in either of two forms, paraformaldehyde or as an aqueous solution (referred to herein as aqueous formaldehyde). Paraformaldehyde is a crystalline solid consisting of a linear polymeric form of formaldehyde of the molecular formula, HO(CH.sub.2).sub.n H where n=8-100. Aqueous formaldehyde consists predominently as formaldehyde in its monomeric form. On standing, it will gradually react with itself forming oligomeric formaldehyde and paraformaldehyde. This is commonly inhibited by adding up to 15% methanol as a stabilizer. The term formaldehyde will be used henceforth to designate that either formaldehyde in the form of paraformaldehyde or aqueous formaldehyde is acceptable (or combinations thereof) unless otherwise specified.
In the above-referenced grandparent patent applications, paraformaldehyde is used to react with isobutyraldehyde to make a product comprising in the case of Ser. No. 691,927, now U.S. Pat. No. 5,144,088, hydroxypivaldehyde (HPA), and in Serial No. 716,177, 3-hydroxy-2,2-dimethylpropyl hydroxypivalate. The products are hydrogenated to make NPG. The two applications describe different catalyst systems for the aldol reaction; the hydrogenation step is distinguished primarily by the fact that hydrogenolysis of the indigenous ester impurities is achieved under relatively mild hydrogenation Conditions. By this process, the feed material for hydrogenation, i.e. the reaction product of the aldol step, after dissolution in a suitable alcohol, may be fed directly to the hydrogenation step and the resultant NPG product may be recovered by distillation. As described in Ser. No. 723,097, this was thought to be due in part to the use of paraformaldehyde as the formaldehyde reactant, which greatly reduces the presence of water and avoids other complications, particularly in the generation of waste. The present application is an improvement on the previous inventions, and in the manufacture of NPG generally, in that the present application recognizes that the hydrogenation process described in Ser. No. 723,097, now U.S. Pat. No. 5,146,012 is admirably suited for input streams with or without the presence of significant amounts of water. References which employ copper chromite and other hydrogenation catalysts with the conventional aqueous formaldehyde system which do not teach the addition of a suitable alcohol solvent prior to hydrogenation may be exemplified by U.S. Pat. No. 4,855,515, which recites the historical development of the reaction and emphasizes the use of a particular catalyst in the hydrogenation step. U.S. Pat. No. 3,808,280 discloses the use of triethylamine as a catalyst for the aqueous formaldehyde/isobutyraldehyde reaction.
Paraformaldehyde is used by Snam S.p.A. in UK Patent No. 1,017,618 to react with IBAL in the presence of a tertiary amine to produce a reaction product containing apparently predominantly HPA which may be hydrogenated to NPG. However, the instant invention teaches the addition of a suitable alcohol solvent prior to hydrogenation or distillation which produces a high purity NPG product by simple distillation, obviating the need for additional expensive purification steps.
While zur Hausen et al, U.S. Pat. No. 4,250,337 may use the aldol reaction product directly in their hydrogenation step, they do not use an alcohol in the hydrogenation step to promote hydrogenolysis. As a result of the use of alcohol, our invention achieves high NPG purities together with high yields unlike the aforementioned patent which can only achieve equivalent purities at uneconomical yields.
Other prior art processes which emphasize the hydrogenation step include U.S. Pat. Nos. 4,094,914 to Rottig et al and 4,933,473 to Ninomiya et al. Ninomiya et al especially recognize the formation of the HPA dimer in the aldol reaction product. Rottig et al use alcohols only in a vapor phase hydrogenation which does not include hydrogenolysis, and do not recognize or demonstrate ester hydrogenolysis.
U.S. Pat. No. 4,855,515 claims the use of a manganese promoted copper chromite which allows for efficient hydrogenation at pressures as low as 500 psig in the presence of the amine catalyst and water. However, the product purification is complicated and expensive. The excess IBAL used in the aldol section must be removed before hydrogenation by distillation. After hydrogenation, the effluent must be treated with caustic to saponify the NPG esters and to neutralize amine salts. The sodium salts formed must then be separated from the product before rectification, and an extractive distillation is also necessary to separate recycle IBAL and triethylamine from water, methanol and isobutanol. The purity of the NPG so derived is not specified.